We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.

Controlling the electrical conductivity of organic semiconductors is a key asset for organic electronics, nowadays realized mostly by molecular dopants. Two doping mechanisms have been reported − chargetransfer complex (CTC) and ion pair (IPA) formation. However, their occurrence depending on molecular structure, energy levels, and structure of thin films remains elusive. Here, we study p-type doping of the planar organic semiconductor dibenzotetrathiafulvalene (DBTTF) in combination with the electron acceptors tetracyanonaphthoquinodimethane (TCNNQ) and hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ) as planar dopants. The conductivity of DBTTF films increases by more than two orders of magnitude upon doping with F6TCNNQ and only slightly with TCNNQ. The highest conductivity is reached at about 10 mol % dopant concentration as a result of two counteracting effects: (1) increasing carrier concentration and (2) reduced carrier mobility due to the growing density of structural defects. We identified two different CTCs for DBTTF:TCNNQ blends and both types of charge-transfer interactions (CTC and IPA) in films of DBTTF doped with F6TCNNQ from absorption measurements. No signature of the charge-transfer interaction is found for DBTTF and TCNNQ in solution, whereas IPA formation only is observed for DBTTF and F6TCNNQ. Many-body perturbation theory calculations of the electronic and optical properties of one-dimensional stacks complement the experimental data and help in understanding the behavior of CTCs. The degree of charge transfer turns out to be higher for the DBTTF:F6TCNNQ complexes than for DBTTF:TCNNQ, as derived from the CN stretching mode softening in infrared absorption. We discuss the different fundamental semiconductor−dopant interactions in solution as compared to the solid state with the aid of the state-of-matter-dependent energy levels of the materials. The presence of both charge-transfer mechanisms in the material combinations investigated here gives us access to their doping efficiency, which is higher for IPA than for CTC formation. Avoiding the CTC formation by structural imperfections seems to be a way to increase the doping efficiency for crystalline materials. The determination of energy levels both in solution and in thin films is beneficial for understanding charge-transfer behavior.

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (J SC ) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C 70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in J SC , elevating the power conversion efficiency by a factor of 1.2 to more than 4 %. FIG. 1. Round crystallites formed in a 50 nm thick DBP-layer by annealing for 10 minutes in chloroform vapor.could be observed. Therefore, in-plane (grazing incident X-ray diffraction) measurements were recorded, but also in this configuration no peaks indicating DBP crystallinity are observable. The results of both, out-of-plane and in-plane measurements are shown in the supplementary material. Other groups have already reported on "more crystalline" DBP, achieved through different techniques. Growing on a crystalline template 38 or on a heated substrate 40 as well as applying organic vapor phase deposition (OPVD) using a hot inert carrier gas 21 were reported to result in DBP layers of higher order. However, in these cases the crystallinity could neither be visualized by means of XRD 38 , nor via reflection high energy electron diffraction 40 (RHEED) nor by selected area electron diffraction 21 (SAED).Next, we investigated the surface properties of 15 nm films of DBP annealed for various durations via AFM. There was no difference for layers growing either on ITO/HIL1.3 or on glass so the reorganization of the molecules is independent of the substrate. These layers were treated by SVA for 4, 8 and 12 minutes, respectively, and then compared with each other as well as with an untreated sample. As reported previously 23,40 , the pristine DBP layer has an extremely smooth surface with a root-mean-square (RMS) roughness of merely RMS = 0.63 nm. However, SVA causes a strong aggregation of the DBP molecules. After 4 minutes of SVA treatment, the RMS roughness increases more than thirtyfold to 21.65 nm. 7 Evaluating the performance of solar cells fabricated with annealed DBP films confirms these problems. For elevate...

We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.

For electron donor–acceptor complexes a link will be established between optical, structural and vibrational properties of EDA complexes as well as the electrical doping by them.